The broad lessons to be drawn from the history of radioastronomy are that large, general-purpose instruments, such as the LT, have dominated the discoveries made by the discipline. By the nature of research, many observing programmes involve working close to current performance limits. Hence raw sensitivity, resulting from a combination of collecting area and the best-available receiving equipment, is a sure-fire route to success. The science case concentrates on the new astrophysical potential which can be unlocked by extending the highest frequency at which the LT's large collecting area can be brought into play. However, it is important to stress that, by securing the long-term viability of the LT, the upgrade will also ensure that the present range of world-leading work can be continued. Some outstanding aspects of this current work are touched on at the start of the main science case.
A common thread, much of it new to LT studies, runs through this future work - the life-cycles of stars in our own Galaxy and throughout the Universe. The upgraded-LT will provide new insights into the many processes involved: from starbirth in collapsing dark clouds of molecular gas, to the collimated jets from single young stars, to the copious chemically-enriched winds from old red-giant stars, to the powerful interactions of stars in binary systems, and finally to the violent explosions seen as novae and supernovae which mark the last phases of the lives of many stars. The embers left behind by supernovae are pulsars, neutron stars spinning many times per second. Supernovae and their remnants, can be studied in distant galaxies and by this means a picture can be built up of the overall history of star-formation over the lifetime of the Universe.
A search for pulsars near the centre of the Galaxy
Dispersion and
multi-path propagation effects arise from the increased density of
ionized gas near the centre of the Galaxy and make searches for
pulsars close to and beyond the galactic centre impossible at the
wavelengths currently accessible to the LT. Working at shorter
wavelengths will allow the LT to be used for searches in the region
where the density of stars is also highest. The pulsars in or behind
this region have a significant probability of being gravitationally
micro-lensed and precise pulse timing can yield strong constraints on
the amount of stellar matter in these regions which are obscured to
visible light.
A survey for faint extragalactic radio sources and new radio stars
Working at 5 GHz with the 25-m Mk2 telescope on the Jodrell Bank site,
the upgraded LT can make a definitive (ten times deeper than is
presently available) large-area survey for faint extragalactic radio
sources. New surveys are the way to discover new things, as was
exemplified by the discovery of the first gravitational lens as a
direct result of survey work carried out with the LT-Mk2
combination. A second definitive survey can be made to even fainter
flux levels in restricted areas, in order to establish the types of
galactic star which show radio emission at a given luminosity.
Studies of stars at different phases in their evolution
An unusual
amount of radio emission is a signature of a star currently undergoing
changes. From the collapse of clouds in starbirth, to the interaction
of binary companions, to the copious chemically-enriched winds from
red giants and finally to a planetary nebula or supernova, MERLIN and
the EVN are ideal instruments for investigating these milestones in
stellar evolution. Radio observations are often the only means of
probing the interaction of stars with their surroundings, which are
usually cool or obscured at other wavelengths by dust clouds.
Stellar radio emission is usually quite weak and hence the scientific
return of current arrays can be greatly increased simply by
increasing their sensitivity. The proposed enhancement of the LT does
exactly that at wavelengths ideal for stellar radioastronomy. The full
science case gives details of the many types of stellar systems which
will be accessible to the LT-enhanced imaging arrays.
Starburst Galaxies and the rate of star-formation in the Universe
Almost all galaxies go through a relatively brief period when they
form large numbers of stars rapidly - the "starburst" phase. The most
massive stars live for a very short time and then explode in supernova
events whose remnants are ideally-suited for study with
high-resolution imaging arrays like MERLIN and the EVN. In the
visible waveband, starburst galaxies are often obscured by dust and
hence are hard to observe. Each starburst galaxy is a laboratory in
which to investigate the formation of massive stars. By measuring the
sizes and expansion rates of supernova remnants, one can place strong
constraints on the current formation rate of massive stars in nearby
galaxies. This calibration of the local universe is important for our
understanding of the rate of star-formation in the early universe
where many galaxies are seen to be undergoing a massive burst of
star-formation. The required high-resolution radio observations are
sensitivity-limited both locally and at high redshift. The addition
of the upgraded LT into MERLIN and the EVN will therefore have a
great impact in this exciting area which is also a target for the Next
Generation Space Telescope and the international mm-wave-array.
Gravitational Lenses
Gravitational lenses provide a unique way of
probing the distribution of matter in the Universe at
cosmologically-important distances. Detailed studies of individual
lens sytems enable the mass distributions of the galaxies acting as
the lenses to be determined. With the aid of this knowledge,
measurements of the time delay between intensity variations in
different lensed images can be used to determine Hubble's Constant
and hence the overall scale of the Universe. For these detailed,
high-resolution studies of gravitational lens systems, imaging
sensitivity is at a great premium. The extra sensitivity provided by
the upgraded LT at the international-standard frequency of 5 GHz will
allow MERLIN and the EVN to make definitive measurements on many of
the known lenses.